I will have my instrument stage check soon, and I will have to be able to explain every instrument as far as to how it functions.

A question I have that nobody has been able to explain to me well is why the directional gyro (heading indicator, if you prefer) needs to have a double gimbal? AFAIK, the gyro only needs to sense yaw, so I don't really get why the need for the other gimbal. I have acces to decomissioned and opened instruments at school to see the internal working if that helps.

BTW, I'm talking for a vacuum driven heading indicator, found in late model C172s.

I guess you could say that. And roll too. If it remains rigid in space the compass card (for the drum type) would remain stable, paralell to the earth's surface where you are flying. So the part of the card that you pointed north (by setting the heading from your wet compass) would remain pointed north. When you make a turn, you are flying the window and lubber line around to different parts of the card - giving you a glimpse of the reading there. If you head south you get to see the part of the card that says "south."

That fact might help you more instinctively understand lead/lag and northerly turning error if those are still taught and tested. It helped me understand them.

If that doesn't make sense leave it alone and go back and re-read something that did.

Happiness is not seeing another trite Ste. Maarten photo all week long.